1. Field of Invention
This invention is directed to controlling pressure at the print head of an inkjet marker.
2. Description of Related Art
Typically, fluid pressure of an inkjet print head is maintained within a tightly controlled range. A negative pressure is usually maintained in an ink tank which is connected to an inkjet print head in order to prevent ink from weeping out of the openings or nozzles of the print head. One conventional method for accomplishing such a negative pressure uses a capillary medium, such as, for example, foam. However, the use of a negative pressure or capillary material, such as, for example, a foam, has a number of drawbacks.
For example, the volumetric efficiency of an ink tank container is reduced by the amount of space which is occupied by the foam or other negative pressure material. Additionally, under relatively high ink flow conditions, the foam or other negative pressure material may create an impedance that raises the negative pressure to relatively unacceptable values, which in turn slows the fluid refill of the inkjet print head nozzles, and may, under certain circumstances, create a “starved jet” condition, i.e., one in which ink is not supplied to the print head in sufficient volume for proper inkjet print head operation. Also, the foam or other negative pressure material in an ink tank is typically only partially saturated and contains a mixture of air and ink which may make it difficult under certain circumstances to prevent a “de-prime” condition. A de-prime condition involves delivery of air out of the foam before useable ink is drained from the foam. Additionally, a foam negative pressure/capillary element may have particles contained therein that may clog inkjet nozzles if the particles reach the inkjet nozzles of a print head.
To address the volumetric efficiency problem, some print heads use a multi-chamber “bubble” design. For example, U.S. Pat. No. 5,182,579 and European Patent 956,959 are examples of bubble design multi-chamber print heads. Multi-chamber bubble design print heads typically have one chamber containing a negative pressure medium, e.g., foam, and other chambers containing only ink. The other chambers which contain ink (and air as the ink is depleted) may be referred to as free ink chambers and are in fluid contact with respect to the negative pressure/capillary medium, e.g., foam, chamber typically through a small opening between the chambers. Ink is withdrawn from the ink tank and provided to a print head by being withdrawn from the chamber with the negative pressure material. Flow of ink between the free ink chamber through the manifold and to the print head, and is pressure controlled as a result of this flow configuration. As ink is depleted from the negative pressure/capillary medium, air enters the inkjet cartridge through a vent, which is typically located above the negative pressure/capillary material. The multi-chamber bubble design ink tank architecture does not, however, solve de-prime or impedance problems mentioned above.
One way to improve the de-prime issue is to drain ink from the ink tank to the print head out of a free ink chamber, not out of a negative pressure/capillary, e.g., foam, chamber. This architecture reduces the de-prime problem, but due to a relatively small area of the opening between the negative pressure/capillary, e.g., foam, material and the free fluid/ink chamber, a relatively high impedance still exists under high flow conditions.
The aforementioned de-prime solution architecture also is not very robust with respect to environmental changes such as, for example, high temperature, and/or low-pressure. For example, when the ink tank temperature is increased or its barometric pressure is increased, any air trapped in the free ink chamber, which typically exists during the mid-life of an inkjet print head and ink tank, will increase/expand in the free ink chamber. This expansion will tend to displace ink and supersaturate the negative pressure material, such as, for example, foam. This super saturation may result in either a positive pressure in the print head and, consequently, weeping of fluid from the nozzles in the print head, and/or the fluid overflowing the foam and causing fluid to leak through the vent opening of the inkjet cartridge, typically located above the foam.
A refillable fluid container system having a pressure control architecture in which persistent air bubbles are released from a capillary or foam fluid reservoir and are directed from an optical level sensing system in a liquid fluid reservoir with which the systems and methods of this invention may be employed is disclosed in copending U.S. patent application Ser. No. 10/747,396, the subject matter of which is hereby incorporated by reference in its entirety.